DC power supply apparatuses which convert an input AC voltage to a DC voltage is used for, for example, an arc welder, an arc cutter, a discharge lamp of an overhead projector, a battery charger and an electroplating apparatus. Some of such DC power supply apparatuses are designed such as to be able to operate from either one of two commercial AC power supplies, e.g. 100 V and 200 V power supplies, one of which supplies a voltage of about one-half the voltage supplied by the other.
One example of such DC power supply apparatus is disclosed in Japanese Examined Patent Publication No. HEI 3-71218. The DC power supply apparatus disclosed in this publication includes an input-side rectifier and smoother circuit, which rectifies an input commercial AC voltage into a DC voltage and smoothes the resulting DC voltage. The smoothed DC voltage is voltage-boosted in a voltage-boosting converter including a semiconductor switching device. The voltage-boosting converter also operates to align the phase of input current to the apparatus with the phase of the input voltage so as to increase a power factor. The output voltage of the voltage-boosting converter is applied to an inverter including semiconductor switching devices, where it is converted to a high-frequency voltage. The high-frequency voltage is then voltage-transformed to a desired voltage by a transformer. The resulting, voltage-transformed, high-frequency voltage is then rectified and smoothed in an output DC providing circuit and is applied to a load.
The DC power supply apparatus of this publication is designed such that it can be operated from an input commercial voltage of 200 V, for example. If a commercial AC voltage of 100 V is applied to this power supply apparatus, the voltage-boosting converter provides the inverter with a boosted voltage which is equal to the voltage which would be supplied to the inverter if a 200 V AC voltage is inputted to the power supply apparatus. Accordingly, the DC power supply apparatus disclosed in Japanese Examined Patent Publication No. HEI 3-71218 can be used in both a 100 V commercial supply voltage system and a 200 V commercial supply voltage system.
There are countries in which both 200 V and 400 V commercial AC voltages are available. If the DC power supply apparatus described above is used in such countries, it can operate normally from a 200 V AC voltage, but it will be damaged when it is operated from 400 V. In order to prevent such apparatus from being damaged even when it is erroneously supplied with 400 V voltage, semiconductor switching devices used in the voltage-boosting converter and the inverter should have a withstanding voltage higher than that of semiconductor switching devices conventional power supply apparatus employ. The use of such high withstanding voltage devices disadvantageously increases the cost of power supply apparatus.
An object of the present invention is to provide a DC power supply apparatus in which semiconductor switching devices having a relatively low withstanding voltage are used, and which does not fail even when an excessive voltage is applied to the apparatus.